Narrow beam electron source for the ion source of a mass spectrometer



H. M. SMITH 3,171,024 TRON SOURCE FOR THE ION SOURCE Feb. 23, 1965 NARROW BEAM ELEC OF A MASS SPECTROMETER Filed Sept. 13. 1961 INVENTOR.

HAYDEN M. SMITH ATT NEY Eoumojamo w on wumaom o 51 WW5 E 0 Emma United States Patent 3,171,024 NARROW BEAM ELECTRON SOURCE FOR THE ION SOURCE OF A MASS SPECTRGMETER Hayden M. Smith, Whitmore Lake, Mich, assignor to The Bendix Corporation, Southfield, Mich, a corporation of Delaware Filed Sept. 13, 1961, Ser. No. 137,818 3 Claims. (Cl. 250-413) This invention relates to an improved electron source for producing a narrow beam of electrons and more particularly to such an electron source for use in time-offiight mass spectrometers.

In the present electron sources, such as are used to ionize gases in mass spectrometers, plates with vertical slits are used with an electron emitter to produce a narrow beam of electrons. Use of a narrow beam is extremely important in certain apparatus. For example, in a time-of-fiight mass spectrometer, a narrow beam reduces the differences in the initial spacing of the ions as they are formed by the beam thus minimizing the error of the spectrometer.

This invention relates to an improved electron source wherein a channel electron multiplier, hereinafter described, is used to produce a narrower beam than has been previously possible with use of the slits mentioned above. The channel electron multiplier is positioned to receive electrons from an emitter, to multiply the electrons and to produce at its output the required narrow beam. Since the electrons from the emitter are amplified, the emitter may be operated at a level below its saturation point to produce the required beam. When operating below saturation, a better range of control can be achieved over the current from the emitter and the electron beam produced at the output of the channel multiplier.

Other objects and advantages will become apparent from the following detailed description and from the appended drawings and claims.

The single drawing is a somewhat schematic diagram partly in block form and partly in prospective, illustrating an embodiment of this invention as used with a timeof-fiight mass spectrometer.

A wedge shaped cathode or filament made from a suitable material, emits electrons when heated. A channel electron multiplier generally indicated at 12 is disposed at a relatively short distance such as .100 inch from the tip of the filament 10 to receive at its input the electrons emitted by the filament.

A tube type channel electron multiplier of the type shown at 12 is fully disclosed in co-pending US. patent application Serial No. 23,574, filed April 20, 1960 by George W. Goodrich and William C. Wiley, now Patent No. 3,128,408. In this type multiplier the inside surface of the tube is conductive and has secondary emissive properties. Upon the application of a voltage ditference between the ends of the tube, current flows through the tube and produces an electric field in an axial direction through the region defined by the tube. Electrons entering the input end of the tube are multiplied through secondary emission before they emerge from the output end of the tube.

The multiplier 12 in FIGURE 1 may be constructed of bismuth-lead oxide glass tubing of small inside diameter such as .005 inch. Initially, the glass tubing is placed in a hydrogen reducing furnace to reduce the bismuth and lead oxides. This converts the tubing from an in sulator to a semiconductor of relatively high resistance. Also, the secondary emissive property of the tube is enhanced. A small length of the tubing, such as .250 inch, is cut oit to form the multiplier 12. The ratio of "Ice the length of the tube to its inside diameter should be approximately 50:1.

A conductive coating 14, such as silver, is applied to the input end surface of the multiplier 12 and a similar coating 15 is applied to the output end surface of the multiplier to provide connecting terminals. A collector 16 is disposed at a relatively great distance, such as 3.94 inches from the output end of the multiplier 12 to receive the electron beam emerging from the multiplier.

A backing plate 18 of a time-of-tlight mass spectrometer is positioned between the multiplier 12 and the collector 16 in perpendicular relationship to the collector. The backing plate 18 is positioned at a relatively short distance, such as .075 inch to the rear of an imaginary line extending from the tip of the filament 10 through the multiplier 12 to the collector 16. An electrode 20 with a horizontal slot 22 is disposed in substantially parallel relationship to the backing plate 18 and at a distance such as .250 inch from the plate. The electrode 20 is positioned in front of the imaginary line mentioned above.

Another electrode 24 with a horizontal slot 26 is positioned in parallel relationship to the electrode 20 and at a distance such as .275 inch from the electrode 20. A detector 28 is positioned at a relatively great distance, such as 14 inches from the electrode 24. An indicator, such as an oscilloscope 30, is connected to the detector 28 to indicate the relative times at which ions of different mass are detected.

Direct voltages such as -1600 volts, l500 volts, 0 volts, 0 volts and vol-ts are applied to the filament 10, the coating 14, the coating 15, the backing plate 18 and the collector 16, respectively, from a direct voltage source 32. Direct voltages, such as +1 volt, 400 volts and 400 volts are also applied to the electrode 20, the electrode 24 and the detector 28, respectively, from the source 32. The voltages applied to the electrode 20 and to the detector 28 are applied through resistances 34 and 36, respectively. I

A negative voltage pulse is applied to the electrode 20 from a suitable pulse source 40. For example, the electrode 20 may be pulsed to a value of 55 volts upon the application of each pulse from the source 40.

It should be understood that the entire apparatus described above operates in a vacuum tube at a pressure of approximately 10- mm. Hg.

During operation electrons emitted by the filament 10 are introduced to the input of the multiplier 12. These electrons are multiplied through secondary emission and emerge from the output of the multiplier as a very narrow beam which passes through the region between the backing plate 18 and the electrode 20 and strikes the collector 16. Molecules of a gas to be analyzed are introduced to the region between the plate 18 and the electrode 20 by suitable means (not shown) and are ionized by the electron beam. The positive ions which are formed are attracted to and are retained by the electron beam because of the charge difference. Since the electron beam is very narrow, the ions are retained in a very narrow region and therefore, any dilterences in their initial positioning are minimized.

Upon the application of each voltage pulse to the electrode 20 from the source 40, the ions retained in the beam are accelerated through the slots 22 and 26 into the field free region between the electrode 24 and the detector 28. The application of the voltage pulse produces an electric field of moderate magnitude in the region between the plate 13 and the electrode 20 and an electric field of considerable magnitude in the region between the electrodes 20 and 24 and a corresponding acceleration of the ions while in these regions. Use of such fields further minimizes any differences in their initial energy 3 as more fully disclosed in US. Patent No. 2,685,035 issued July 27, 1954 to William C. Wiley. When the ions reach the field free region, they drift until they strike the detector 28. Because of their differences in travel times, ions of different mass strike the detector 28 at different times as indicated on the oscilloscope 30.

Although the electron source comprising the filament and the multiplier 12 has been shown as used in a time-of-flight mass spectrometer, it will be recognized by persons skilled in the art that this electron source can be used in other apparatus where it is important to provide an extremely narrow beam.

The use of the channel multiplier 12 produces an extremely narrow beam not heretofore possible. Also, be-

cause the electrons emitted by the filament 10 are ampli-- fied by the. multiplier 12, it isnot necessary to operate the filament 10 at saturation to produce an electron beam of sutficient energy. Therefore, it may be operated at a level below saturation thus permitting better control of the electron output of the filament and the intensity of the beam produced at the output of the multiplier 12.

Although this invention has been disclosed and illustrated with reference to particular applications, the principles involved are susceptible of numerous other applications which will be apparent to persons skilled in the art. indicated by the scope of the appended claims.

Having thus described my invention I claim: 1. In combination, a backing plate, a first electrode disposed relative to the backing plate to define a first region between the first electrode and the backing plate,

a second electrode disposed relative to the first elec-- trode to define a second region between the first and second electrodes,

means for emitting electrons,

an electron multiplier consisting solely of a tube positioned to receive emitted electrons into one end of the tube and to provide a narrow electron beam emerging from the opposite end of the tube,

said electron beam being directed through said first region to ionize molecules of a gas in said first region,

said tube having its longitudinal dimension substantially greater than its diameter,

said tube having a secondary electron emissive inner surface which is conductive,

means for producing an electrical current flow between the ends of said tube through said secondary electron emissive surface to establish an electric field in the region defined by the tube,

means for providing an electric field of' particular magnitude in said first region to move the ions past the first electrode into the second region,

means for providing an electric field of a magnitude greater than said particular magnitude in the second region to move the ions past the second electrode,

means disposed relative to the second electrode to detect the ions moving past the second electrode,

The invention is, therefore, to be limited only as and means for indicating the relative times at which ions of different mass reach said detecting means.

2. In combination,

a backing plate,

an electrode disposed relative to the backing plate to define a region between the electrode and the backing plate,

means for emitting electrons,

an electron multiplier consisting solely of a tube positioned to receive the emitted electrons into one end of the tube and to provide a narrow electron beam emerging from the opposite end of the tube,

said electron beam being directed through said region to ionize molecules of a gas in said region,

said tube having its longitudinal dimension substantially greater than its diameter,

said tube having a secondary electron emissive inner surface which is. conductive,

means for producing an electrical current flowbetween the ends of said tube through said secondary electron emissive surface to establish an electric field in the regiondefined by the tube,.

means for providing an electric field in said region to move the ions past the, electrode,

means disposed relative to the electrode to detect the ions moving past the electrode,

and means for indicating the relative times at which ions of different mass reach said detecting means.

3. In combination,

means defining a region containing molecules of a gas,

means for emitting electrons,

an electron multiplier consisting solely of a tube positioned to receive the emitted electrons into one end of the tube and to provide a narrow electron beam emerging from the opposite end of the. tube,

said electron beam being directed through said region to ionize the molecules in said region,

said tube having its longitudinal dimension substan- References Cited by the Examiner UNITED STATES PATENTS 6/40 Farnsworthret al. 313-68 8/40 Keyston 315-12 9/56 Wiley 250-41.91 10/56 Wiley 250-4l.9l

RALPH G. NILSON, Primary Examiner. 

3. IN COMBINATION, MEANS DEFINING A REGION CONTAINING MOLECULES OF A GAS, MEANS FOR EMITTING ELECTRONS, AN ELECTRON MULTIPLIER CONSISTING SOLELY OF A TUBE POSITIONED TO RECEIVE THE EMITTED ELECTRONS INTO ONE END OF THE TUBE AND TO PROVIDE A NARROW ELECTRON BEAM EMERGING FROM THE OPPOSITE END OF THE TUBE, SAID TUBE HAVING ITS BEAM DIRECTED THROUGH SAID REGION TO IONIZE THE MOLECULES IN SAID REGION, SAID TUBE HAVING ITS LONGITUDINAL DIMENSION SUBSTANTIALLY GREATER THAN ITS DIAMETER, SAID TUBE HAVING A SECONDARY ELECTRON EMISSIVE INNER SURFACE WHICH IS CONDUCTIVE, AND MEANS FOR PRODUCING AN ELECTRICAL CURRENT FLOW BETWEEN THE ENDS OF SAID TUBE THROUGH SAID SECONDARY ELECTRON EMISSIVE SURFACE TO ESTABLISH AND ELECTRIC FIELD IN THE REGION DEFINED BY THE TUBE, MEANS FOR EXTRACTING THE IONS FROM SAID REGION, MEANS DISPOSED RELATIVE TO SAID REGION TO DETECT THE IONS EXTRACTED FROM THE REGION, AND MEANS FOR INDICATING THE RELATIVE TIMES AT WHICH IONS OF DIFFERENT MASS REACH SAID DETECTING MEANS. 